In the related art, there is a sewing machine configured to do a sewing such as embroidery (hereinafter, referred to as “free-motion sewing”) at a given position and in a direction on a sewing object by moving the sewing object freely. In order to do embroidery or the like at an arbitrary position, a mechanism to move the sewing object arbitrarily is necessary. An upper thread is connected to the sewing object in a course of doing embroidery. Generally, in the sewing machine, a structure in which the upper thread is constantly tensed is used, and moving the sewing object against the tension is not comfortable.
Accordingly, a mechanism for releasing the tension of the upper thread manually when moving the sewing object is provided. However, in view of an improvement of convenience and prevention of forgetting of tension restoration, automation of release of the tension of upper thread is required.
For example, in WO2010/109773 (Reference 1), as a user assistance function in the free-motion sewing using a computer sewing machine, a stitching width can be adjusted in accordance with a pressing amount of a foot controller and, in addition, the tension of the upper thread is automatically released without operating a holder lift lever when the sewing machine is stopped. When the foot controller is pressed, the tension of the upper thread is automatically restored.
Since a resistance force caused by the tension of the upper thread is not generated when a user moves a process fabric during embroidery, the user is allowed to work with comfort without any stress. In order to realize the above-described function, an electric actuator configured to open and close a tension disc is controlled.
However, since the sewing machine of Reference 1 has a structure of the general computer sewing machine added with an “upper thread tension adjusting mechanism” composed of the electric actuator, increase in manufacture cost of a sewing machine body and increase in weight of the product are resulted.